1. Field of the Invention
The present invention relates to a developing device for an image forming device, such as a copy machine, a laser printer, or a facsimile machine. More particularly, the invention relates to a toner supply roller for supplying toner particles to a developing roller which in turn supplies the toner particles to a photosensitive drum.
2. Description of the Related Art
A developing device provided in an image forming device, such as a copy machine, a laser printer, or a facsimile machine, includes: a photosensitive drum on which an electrostatic latent image is formed; a developing roller for bearing toner particles thereupon and supplying toner particles to the photosensitive drum; and a supply roller for supplying toner particles to the developing roller. The developing roller is disposed in contact with the photosensitive drum. The supply roller is formed from a foam material and is disposed in contact with the developing roller so that the foam material is compressed by a certain amount.
To perform printing, a charge unit uniformly charges the surface of the photosensitive drum. A laser generating unit then irradiates the charged surface of the photosensitive drum by scanning a laser beam across the surface of the photosensitive drum. The laser beam forms an electrostatic latent image on the surface of the photosensitive drum based on image data.
The supply roller is rotated to transport toner particles from a toner box to the developing roller. The toner particles are charged by rubbing contact with the supply roller and the developing roller where the two rollers contact each other.
Toner particles supplied onto the developing roller are regulated into a thin film on the surface of the developing roller by a blade, for example, disposed at the side of the developing roller. The developing roller is applied with a bias voltage having a fixed voltage value so that toner borne on the developing roller travels from the developing roller toward the electrostatic latent image formed on the photosensitive drum.
Therefore, when rotation of the developing roller brings the charged toner particles borne on the developing roller into contact with the photosensitive drum, the charged toner particles cling to the electrostatic latent image formed on the surface of the photosensitive drum so that the electrostatic latent image is developed by the toner particles into a visible toner image. Afterward, toner particles clinging to the surface of the photosensitive drum are transferred onto a sheet, thereby completing the printing process.
In order for this type of developing device to properly develop an electrostatic latent image using toner particles, the toner particles must be sufficiently charged. When insufficiently charged toner particles on the developing roller are used to develop the electrostatic latent image, the insufficiently charged toner particles can cling to areas of the photosensitive drum other than areas formed with the electrostatic latent image, so that the resultant toner image will be different from the desired image.
To insure that the toner particles supported on the developing roller are sufficiently charged, the toner particles must be brought into sufficient contact with the supply roller and the developing roller where the supply roller and the developing roller contact each other, so that the toner particles can be sufficiently charged by friction between the supply roller and the developing roller. This requires that the supply roller be strongly pressed against the developing roller. However, if the supply roller presses too strongly against the developing roller, the developing roller can be damaged so that lines and streaks, and other signs of poor image development, can appear in the developed image.